Devices based on surface plasmon interference filters

ABSTRACT

Devices based on surface plasmon filters having at least one metal-dielectric interface to support surface plasmon waves. A multi-layer-coupled surface plasmon notch filter is provided to have more than two symmetric metal-dielectric interfaces coupled with one another to produce a transmission spectral window with desired spectral profile and bandwidth. Such notch filters can form various color filtering devices for color flat panel displays.

This application is a continuation-in-part of the U.S. patentapplication Ser. No. 08/949,151, filed on Oct. 10, 1997, now U.S. Pat.No. 5,986,808. This application further claims the benefit of the U.S.Provisional Application No. 60/056,050, filed on Sep. 2, 1997, No.60/059,247, filed on Sep. 18, 1997, and No. 60/060,733, filed on Oct. 1,1997.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The present invention relates to wavelength tunable optical filters and,more particularly, to devices based on a surface plasmon tunable filter.

BACKGROUND

An optical wavelength filter is a device that reflects or transmitslight of a desired wavelength or within a certain wavelength range. Forexample, an interference bandpass filter selectively transmits lightwithin a selected wavelength transmission bandwidth while absorbinglight of wavelengths outside the transmission bandwidth. Such opticalfiltering with respect to wavelength provides a means of controlling theenergy and spectral composition of light and is widely used in a varietyof optical signal processing, detection, and display applications.

Excitation of surface plasmon waves at a metal-dielectric interface hasbeen demonstrated as an efficient way of implementing a spectralfiltering mechanism in response to an electrical control signal. See,for example, Wang and Simon, “Electronic Reflection with SurfacePlasmon,” Opt. Quantum Electron.25, S925 (1993) and Wang,“Voltage-Induced Color-Selective Absorption with Surface Plasmon”, Appl.Phys. Lett. 67, pp. 2759-2761 (1995). Surface plasmon are oscillationsof free electrons caused by resonant absorption of a p-polarizedincident optical wave at a metal-dielectric interface when thewavelength and incident angle of the optical wave satisfy a plasmonresonance condition. More specifically, the plasmon resonance conditionrequires that the component of the optical wave vector along themetal-dielectric interface matches the plasmon wave vector, K_(p):$\begin{matrix}{{K_{p} = {\frac{2\pi}{\lambda}\sqrt{\frac{\varepsilon_{1}\varepsilon_{2}}{\varepsilon_{1} + \varepsilon_{2}}}}},} & (1)\end{matrix}$

where, ∈ is the wavelength of the optical wave, ∈₁ and ∈₂ are thedielectric permittivity constants for the metal and the dielectricmaterial, respectively.

At surface plasmon resonance, the energy of the incident optical wave isstrongly absorbed and converted into the energy of oscillating freeelectrons in the metal. Therefore, the reflected optical wave isstrongly attenuated or even vanishes. When the incident angle of theoptical wave is fixed at a constant, the optical wavelength λ satisfyingthe plasmon resonance condition may be changed by varying the dielectricpermittivity constant ∈₂ of the dielectric material. If the inputoptical wave is white light, the color of the reflected optical wavewill change with ∈₂. This phenomena effects a surface plasmon tunablefilter in reflection mode.

Therefore, an electronically tunable filter can be formed by using anelectro-optic material as the dielectric material. The voltage appliedon the electro-optic material changes its index of refraction andthereby changes the wavelength for the surface plasmon resonance.

Wang and Simon disclose color display devices based on a surface plasmonfilter using a liquid crystal electro-optic material. U.S. Pat. Nos.5,451,980 and 5,570,139, which are incorporated herein by reference. Theindex of the refraction of the liquid crystal is changed by applying avoltage to alter the spectral composition of the reflected light.

SUMMARY

The devices disclosed herein use surface plasmon waves atmetal-dielectric interfaces to alter the spectral composition of lighthaving a p-polarized component. The metal material in general has anegative dielectric constant and the dielectric material has a positivedielectric constant. The electrical field of the p-polarized componentat non-normal incidence induces electric dipoles in a metallic layerthat forms one side of a metal-dielectric interface due to theexcitation of the free electrons in the metal. The direction of theinduced dipoles is perpendicular to the metal-dielectric interface. Theradiation of the dipoles generates a surface plasmon wave with a wavevector parallel to the interface. The strength of the surface plasmonwave is maximal at the metal-dielectric interface and decaysexponentially on both sides of the interface.

The energy conversion from the incident light to the surface plasmonwave is maximal when the incident angle, wavelength of the incidentlight, the dielectric constants of the metal and the dielectricmaterials satisfy a surface plasmon resonance condition. In general,this resonance condition relates to mode matching between thep-polarized incident light and the surface plasmon wave at ametal-dielectric interface and may vary with the specific incidentcoupling mechanism and the structure of the interfaces (e.g., a singleinterface or two closed coupled interfaces).

One embodiment of a surface plasmon filter includes a dielectric layersandwiched between two metallic layers to form two closely spacedsymmetrical metal-dielectric interfaces. The optical thickness of thedielectric layer is configured to allow for excitation of surfaceplasmon waves on both metal-dielectric interfaces by an input opticalwave. The dielectric layer may be less or larger than one wavelength butin general on the order of a wavelength. The coupling between thesurface plasmon waves produces a reflected wave and a transmitted wavethat have mutually complimentary colors.

The surface plasmon resonance frequency can be tuned by adjusting theoptical thickness of the dielectric layer. Either the layer thickness orthe index of the refraction of the dielectric layer may be adjusted tochange the transmission wavelength. One implementation uses anadjustable air gap as the dielectric layer. Another implementation usesa layer of an electro-optic material to vary the optical thickness bychanging the index of refraction with a voltage control signal.

One or more additional metal-dielectric interfaces may be added andcoupled to the two metal-dielectric interfaces to form a multilayersurface plasmon filter. Such a multilayer structure can be configured toachieve a desired shape in the transmission spectrum profile. Forexample, a “notch” filter can be so formed to produce a square-liketransition from a transmissive spectral region to a reflective spectralregion and to achieve a desired transmissive bandwidth.

The surface plasmon filter can be used to form a wide range of devices.One such device is a tunable Fabry-Perot filter based on an air-gapsurface plasmon filter. Various color filters for color display systemssuch as color LCD displays can be formed based on a surface plasmonfilter.

These and other aspects and advantages of the present invention willbecome more apparent in light of the accompanying drawings, the detaileddescription, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing one embodiment of the surface plasmonfilter using a high-index material for light coupling.

FIG. 1B is a diagram showing another embodiment of the surface plasmonfilter using a grating for light coupling.

FIG. 2 is diagram showing a surface plasmon filter using a layer ofelectro-optic material and high-index prisms.

FIG. 3A is a chart of calculated transmission spectrum of the surfaceplasmon filter of FIG. 2 using silver films under different changes inthe index of refraction of the electro-optic layer.

FIG. 3B is a chart of calculated transmission spectrum of the surfaceplasmon filter of FIG. 2 using potassium films under different changesin the index of refraction of the electro-optic layer.

FIG. 4 is diagram showing a surface plasmon filter using an adjustableair gap and high-index prisms.

FIG. 5 is a chart of calculated transmission spectrum of the surfaceplasmon filter of FIG. 4 under different spacings of the air gap.

FIG. 6 is diagram showing a surface plasmon filter formed with two glassplates having micro-prisms arrays.

FIG. 7 is a chart of calculated transmission spectrum of the surfaceplasmon filter of FIG. 6 under different spacings of the air gap betweenthe glass plates.

FIG. 8A is a block diagram showing a high-resolution spectrometer basedon a Fabry-Perot filter and a surface plasmon filter.

FIG. 8B is a diagram illustrating integration of a surface plasmonfilter and a sensor array.

FIG. 9 is a diagram of color LCD display based on a surface plasmonfilter.

FIG. 10 is a diagram showing one embodiment of a multi-layer surfaceplasmon filter having six coupled metal-dielectric interfaces.

FIG. 11 shows calculated reflection spectrum of the filter of FIG. 10with three identical liquid crystal layers of 185 nm thick, two outersilver films of 20 nm thick and two middle silver films of 40 nm thick.

FIG. 12 is a flowchart showing the design process of a multi-layersurface plasmon notch filter

FIG. 13A are plots showing exemplary colors that can be generated bydifferent overlapping the reflection spectra of two tunable surfaceplasmon filters.

FIG. 13B is a diagram showing one embodiment of a reflective flat panelcolor display based on the technique shown in FIG. 13A.

FIG. 13C is a diagram showing one implementation of a reflective flatpanel color display of FIG. 13B.

FIG. 14A is a diagram showing one embodiment of a color filtering devicefor producing sequential colors.

FIG. 14B F shows the reflectivity spectra of three filters based on amulti-layer design when illuminated by a white input beam.

FIG. 14C is a CIE diagram to show the color purity of the primary colorsproduced by the filters shown in FIG. 14B.

FIG. 15 is a diagram illustrating a color display system based on thefilter of FIG. 14A.

FIG. 16A is a diagram showing an exemplary smart card image device basedon surface plasmon filters.

FIG. 16B illustrates the manufacturing process of the smart card imagingdevice of FIG. 16A.

FIG. 16C is a diagram showing one design of addressing the transistorsin the device of FIG. 16A.

FIGS. 17A, 17B, and 17C show structure and operation of one color filterusing three surface plasmon tunable filters.

FIG. 18 shows an another color filter based on the device of FIGS. 17Athrough 17C.

FIG. 19A shows an alternative construction of the pixel in the device ofFIG. 16A, where only three layers are used to form a surface plasmonfilter: a metal layer, a liquid crystal layer and a thin-film transistorlayer.

FIG. 19B shows that each plate can be a simple high-index glass platewith one side formed with thin-film transistors and the other sidecoated with a metal layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A shows one embodiment 100 of a coupled surface plasmon tunablefilter. Two symmetrical metal-dielectric interfaces 110 a and 110 b areformed by sandwiching a dielectric layer 110 between two substantiallyidentical metallic layers 120 a and 120 b. The metallic layers 120 a and120 b are sufficiently thin so that the evanescent electromagnetic wavescan penetrate the metallic layers 120 a and 120 b. The evanescent wavesmay be generated by, for example, an incident optical wave or couplingof oscillating electrical fields caused by surface plasmon waves. Ingeneral, the thickness of the metallic layers 120 a and 120 b may be anyvalue. However, the preferred thickness is from about 5 nm to about 150nm and most preferably from about 10 nm to about 100 nm. A variety ofmetals may be used for forming the layers 120 a and 120 b, including butnot limited to, Ag, Al, Au, K, and Li.

The dielectric layer 110 may be any dielectric material, including airor an electro-optic material including inorganic crystals (e.g., KDP,KTP, LiNbO₄), polymide guest-host materials, organic crystals (e.g.,MMONS and MNA), organic salts, and liquid crystals. The thickness of thedielectric layer 110 is sufficiently thin to allow for efficient energycoupling between the first metallic-dielectric interface 110 a and thesecond metallic-dielectric interface 110 b. Preferably, the opticalthickness of the dielectric layer 110 is on the order of a wavelength.

An incident optical wave 102 to the metallic layer 120 a can excite asurface plasmon wave at the first interface 110 a if the surface plasmonresonance condition is met. The energy of photons in the surface plasmonresonance is converted into collective oscillations of free electronsgenerated in the first metallic layer 120 a. This causes strongabsorption of the photons at the resonant wavelength in the incidentoptical wave 102 and the unabsorbed photons are reflected as thereflected wave 104 from the first metallic layer 120 a.

The oscillating free electrons in the first metallic layer 120 agenerate an evanescent optical wave at the wavelength of the absorbedresonant photons. The field of the evanescent optical wave penetratesthe thin dielectric layer 110 to reach the second metallic-dielectricinterface 110 b. The field of the evanescent optical wave can excite asecond surface plasmon wave on the second interface 110 b. This is atleast in part due to the symmetry of the two interfaces 110 a and 110 bwith respect to the dielectric layer 110. The second surface plasmonwave is substantially identical to the first surface plasmon waveincluding the frequency and the plasmon wave vector. The oscillatingfree electrons in the second metallic layer 120 b radiate photons in thesame direction and at the same frequency as the absorbed resonantphotons at the first metallic layer 120 a. The radiated photons exit thesecond metallic layer 120 b as a transmitted wave 106 which issubstantially parallel to the input beam 102.

Therefore, for an incident light with a broad spectrum, the device 100of FIG. 1 will couple the spectral component that satisfies the surfaceplasmon resonance condition to the second interface 110 b as thetransmitted wave 106 and reflects the rest of the input light as thereflected wave 104. In particular, a white input beam may be split intoa colored transmitted beam and a reflected beam which is spectrallycomplimentary to the transmitted beam.

The excitation of the first surface plasmon wave at the first interface110 a and the excitation of the second plasmon wave at the secondinterface 110 b are correlated. Under a surface plasmon resonancecondition, the evanescent field pattern of the first surface plasmonwave in the dielectric layer 110 and the evanescent field pattern of thesecond surface plasmon wave affect each other to match the fieldpatterns. The incident angle and wavelength of the incident optical wave102 and the optical thickness of the dielectric layer 110 can beadjusted to satisfy the resonance condition in order to achievesimultaneous excitation of both the first and the second plasmon waves.The exact resonance condition can be determined by applying the Maxwellwave equations to each of the dielectric layer 110 and the two metalliclayers 120 a and 120 b and by matching the boundary conditions atinterfaces 110 a and 110 b.

For a given incident angle of the input optical wave 102, the opticalthickness of the dielectric layer 110 may be adjusted to achieveexcitation of the surface plasmon waves at different wavelengths toeffect color filtering in both transmitted beam 108 and the reflectedbeam 104. The optical thickness of the dielectric layer 110 is theproduct of its index of refraction and the layer thickness. Therefore,the layer thickness of the dielectric layer 110, or the index ofrefraction, or both may be varied in order to select differentwavelengths to meet the resonance condition of surface plasmon waves atthe interfaces 110 a and 110 b.

Referring to FIG. 1, the embodiment 100 further includes two identicaldielectric layers 130 a and 130 b respectively formed on the metalliclayers 120 a and 120b. The index of the refraction of the layers 130 aand 130 b is larger than that of the dielectric layer 110 in order toproperly couple the input optical beam 102 to excite surface plasmonwaves. For example, a high-index prism (e.g., TiO₂) may be used tocouple the input optical wave 102 at a specified incident angle. Ingeneral, the incident angle of the input optical wave 102 is larger thanthe critical angle for total reflection defined by the high-index layer130 a (or 130 b) and the dielectric layer 110.

The coupling dielectric layers 130 a and 130 b are usually in form ofprisms and may be made of any dielectric material with an index ofrefraction higher than that of the dielectric layer 110. Examples ofsuitable high-index materials include glasses such as SF glass (e.g.,SF6, SF57, SF 58, SF 59, etc.) and LaSF glass (e.g., LaSFN18, LaSFN31,LasFN9, LaSF13, etc.), crystals such as TiO₂, sapphire, diamond, andhigh-index polymers.

Alternatively, a grating may also be used for light coupling as shown inFIG. 1B. In this embodiment, two substantially identical gratings 140 aand 140 b substitute the high-index coupling dielectric layers (130 aand 130 b). Each of the gratings 140 a and 140 b is coated with a layerof metal film (150 a and 150 b) to form the metal-dielectric interfaces.The gratings 140 a and 140 b are configured to produce a diffractedorder 102 b of the incident light 102 to have a wave vector parallel tothe metal-dielectric interfaces. For example, the first-orderdiffraction beam may be used as the beam 102 b to excite a surfaceplasmon wave and the zero-order diffraction beam is reflected as thebeam 102 a. Similar to the embodiment 100 of FIG. 1A, the couplingbetween the two symmetric interfaces produces the transmitted beam 106.

In the case where a surface plasmon wave is excited at only onemetal-dielectric interface, the resonance condition requires$\begin{matrix}{{{\frac{2\pi}{\lambda}\sin \quad \Theta} \pm {\frac{2\pi}{d} \cdot p}} = K_{p^{\prime}}} & (2)\end{matrix}$

where d is the grating period, p is an integer indicating the order ofdiffraction and K_(p) is the surface plasmon wave vector defined byEquation (1). For the coupled symmetric structure shown in FIG. 1B, thesurface plasmon resonance condition is more complex than Equation (2)and can be similarly determined as in the embodiment 100 of FIG. 1A byapplying the Maxwell wave equations to each of the dielectric layer 110and the two metallic layers 150 a and 150 b and by matching the boundaryconditions at the interfaces.

The gratings 140 a and 140 b may be implemented in a number of ways. Forexample, one way is to attach a separately-formed grating to thedielectric layer 110; another way is to directly form a coupling gratingon the dielectric layer 110; yet another way is to etch a grating on thesurface of the dielectric layer. The gratings 140 a and 140 b in firsttwo examples are preferably made of a transparent material in theoperating spectral range. A dielectric material may be used to form asupport for the grating coupled filter of FIG. 1B.

One implementation of the embodiment 100 is shown in FIG. 2. A thinlayer of electro-optic material 210 with a fixed thickness less than awavelength is sandwiched between two thin metallic films 220 a and 220b. A first prism 230 a is used as an input light coupling element byplacing the hypotenuse in contact with the first thin metallic film 220a. Symmetrically, a second prism 230 b identical to the 230 a isdisposed on the second metallic film 220 b as an output light couplingelement. The index of refraction of the prisms 230 a and 230 b is largerthan that of the electro-optic material 210. An electrical voltagesupply 212 is connected to the electro-optic material 210 to provide anelectrical control of its index of refraction.

The electro-optic material 210 may be any suitable electro-opticmaterial including KDP, KTP, LiNbO₄ crystals or a liquid crystalmaterial. The thin metallic films 220 a and 220 b may be made of silver,aluminum, or other metals with desired dispersion properties in thesurface plasmon excitation. In the visible spectral range, metals suchas Ag or Al may be used. In the IR range, Au and alkali metals such as Kor Li may be used. The prisms 230 a and 230 b may be made of ahigh-index glass as previously described.

The index of refraction of the electro-optic material 210 changes inresponse to a voltage control signal from source 212. This index changefurther causes a change in the optical thickness of the electro-opticmaterial 210. Therefore, the surface plasmon resonance frequency and thetransmission spectrum of the device 200 change accordingly.

FIG. 3A is a chart showing the calculated transmission spectrum for twosilver films separated by a 150-nm electro-optic material layer. Thecalculation is based on the Maxwell wave equations. The prisms are madeof TiO₂ and the metallic films 220 a and 220 b are 35-nm silver films.The incident angle is fixed at 45°. When no voltage is applied, theindex change dn is zero, and the peak transmission is about 62% at 450nm (blue). When the voltage-induced index change of the electro-opticlayer is dn =0.2, the transmission peak shifts to 530 nm (green) with atransmission of about 73%. When the index increases by an amount of dn=0.5, the peak transmission shifts to 650 nm (red) with a transmissionof about 70%. Thus, all three primary colors (red, green and blue) canbe achieved in the transmitted light by changing the index of theelectro-optic layer by an amount in a range of from 0 to about 0.5.

The intensity and linewidth of a transmission peak in the filter 200depend on the optical properties and the thickness of the metallic films220 a and 220 b. Metals with small imaginary part of the dielectricconstant usually lead to higher peak transmission and narrowerbandwidth. A thinner metallic layer can be used to achieve greater peaktransmission and broader bandwidth.

In addition, different operation spectral ranges may be achieved byusing different metals for the metallic films 220 a and 220 b. Forexample, potassium films may be used to replace the silver films in thedevice of FIG. 2 to change the transmission spectrum from the visiblerange to the infrared (IR) range. FIG. 3B shows that potassium filmseach of 900 Å are used to achieve a tunable IR spectral range from 1050nm to 1700 range by varying the index of the electro-optic layer 210 byan amount in a range of from 0 to about 0.5, i.e., index varies fromabout 1.5 to about 2.0. The coupling prisms are made of TiO₂ and theincident angle is about 43°. This particular IR range covers thetransmission windows near 1.3 μm and 1.5 μm for fiber communicationsystems.

Another implementation of the embodiment 100 is shown in FIG. 4 whichreplaces the electro-optic material 210 of FIG. 2 with an adjustable airgap 410 between the two prisms 230 a and 230 b. In this configuration,the metallic films 220 a and 220 b are respectively formed on thehypotenuses of the prisms. The metallic-dielectric interfaces along withthe surface plasmon waves are excited are metal-air interfaces. Thespacing between the metallic films 220 a and 220 b may be varied by apositioning device such as a piezo-electric transducer. FIG. 5 shows thecalculated transmission spectrum of the device of FIG. 4 under differentspacings of the air gap 410. The calculation is based on an incidentangle of 40°, silver films of 400 Å, and coupling prisms made of the BK7glass. As the air gap 410 increases from about 200 nm to about 750 nm,the transmission peak shifts from about 400 nm to about 700 nm withinthe visible spectral range. Transmissions at other spectral ranges mayalso be achieved, for example, by using different metallic films 220 aand 220 b.

The filter configuration shown in FIG. 4 can be used to form a tunablecolor filter array by using two micro-prism plates. An example is shownin FIG. 6. Two “T” shaped glass plates 610 and 620 each have a pluralityof micro-prisms 612 and 622 arranged in a one-dimensional ortwo-dimensional prism array on one side of the plates. The micro-prisms612 and 622 may be formed by etching or other microprocessingtechniques. A thin metal film layer is coated on the micro-prisms oneach glass plate. The filter 600 is formed by placing the two glassplates 610 and 620 together with the micro-prism sides conforming witheach other. Two adjustable spacers 630 (e.g., voltage-controlledpiezo-electric spacers) are used to separate the two glass plates 610and 620 by a thin air gap 640 between the micro-prisms 612 and 622.Preferably, the thin air gap 640 is less than a wavelength. As thelength of the spacers 630 is adjusted, the air gap 640 between themicro-prisms 612 and 622 changes. This results in a change in thesurface plasmon resonance frequency and consequently a shift in thewavelength of the transmission spectrum.

FIG. 7 shows the calculated transmission of the device of FIG. 6 as afunction of wavelength for various spacings of the air gap 640. Thecalculation is based on silver films of 40 nm thick, an incident angleof 42.5°, and coupling prisms made of the BK7 glass. When the air gap640 increases form 300 nm to 5000 nm, the peak reflectivity shifts from400 nm to 1600 nm. The transmission peaks as labeled are: peak 710 at anair gap of 300 nm, peak 720 at an air gap of 800 nm, peak 730 at an airgap of 1500 nm, peak 740 at an air gap of 3000 nm, and peak 750 at anair gap of 5000 nm.

The air gap surface plasmon tunable filter shown in FIGS. 4 and 6 can beoperated under a wide range of temperatures. For example, an operatingtemperature range from about −200° C. to +200° C. can be achieved byusing piezo-electric spacers and choosing the prism glass to match thethermal expansion of the piezo-electric spacers.

Alternatively, the air gap 640 in the micro-prism surface plasmon filter600 of FIG. 6 may be replaced by a layer of electro-optic material witha fixed thickness. The surface plasmon resonant frequency can be alteredby electrically changing the index of refraction.

The above-described surface plasmon tunable filters may combine with aFabry-Perot filter to form a high-resolution spectrometer as shown inFIG. 8A. Fabry-Perot filters can be made to have a high fineness numberto achieve high spectral resolution. However, the tuning range of manyFabry-Perot filters is limited. The spectrometer of FIG. 8A has anadvantage of the high resolution of the Fabry-Perot to filter and thewide tunable range of the surface plasmon filter.

FIG. 8B shows another device based on the surface plasmon filter. Thisdevice combines the micro-prism surface plasmon filter 810 with a sensorarray 820. The micro-prism surface plasmon filter 810 has an activelayer 812 which can be either an air gap or a layer of electro-opticmaterial between two metallic films. The sensor array 820 may be anyphotodetector array including a CCD array, a diode array, aphototransistor array or an active pixel sensing array (“APS”). Inoperation, the surface plasmon filter 810 selects the transmissionspectrum and the sensor array 820 measures the intensity of the incidentpattern, respectively. As shown in FIG. 8A, a Fabry-Perot filter may beadded to the device of FIG. 8B to enhance the spectral resolution.

In addition, the device of FIG. 8B may be integrated on a single chip toform an image spectrometer-on-a-chip. In particular, an APS array may beused as the sensor array and the active layer 812 of the surface plasmonfilter may be divided into many pixels to match the pixel size of theAPS array. The pixel of the active layer 812 can be addressed by anactive matrix and the spectrum of each pixel can be individuallyadjusted.

Furthermore, a surface plasmon filter can be used in either a projectionor a direct-view color display system. FIG. 9 shows a back-lit colorliquid crystal display (“LCD”) system 900 based on a prism-coupledsurface plasmon filter 930. A light source 910 and a lens system 920produce a white beam 922 with a substantially homogenous illuminationprofile. The light source 910 may be a lamp with a line-shaped filamentand the lens system 920 may simply be a cylindrical lens. The surfaceplasmon filter 930 receives the white beam 922 and produces a beam 924.The color of the beam 924 is electrically controlled by the filter 930using either an adjustable air gap or an electro-optic material. Areflector 940 further guides the filtered beam 924 to a monochromaticLCD panel 950. Preferably, the reflective surface of the reflector 940has structures to make the reflected light diffusive in order to improvethe homogeneity of the images.

The filter 930 is configured to operate in the visible spectral rangefrom about 400 nm to about 750 nm. For example, the prisms may be madeof TiO₂ and the metallic films may be made of silver. In addition, anelectro-optic material with an index variation range of about 0.5 may beused to achieve all three additive primary colors (i.e., red, green andblue). FIG. 3A shows one possible spectral output of such a filter.

The filter 930 and the LCD panel 950 are controlled by a display controlcircuit 960. The filter 930 performs color filtering in the homogeneousillumination beam 924 and the LCD panel 950 modulates the intensitydistribution of the beam 924 to form images. During each frame scanningin the LCD panel 950 (e.g., 60 frame scans per second), the controlcircuit 960 controls the filter 930 to change the color of the beam 924three times by sequentially hopping through three primary colors. Thisprocess produces colored images. In many LCD color displays, threepixels are used to produce one image pixel and each pixel has a colorfilter to produce one of the three primary colors. Thus, the LCD display900 of FIG. 9 eliminates the color filter in each pixel and can achievethe same resolution in the color images by using only one third of theactive pixels in the conventional LCD color displays. In addition, theLCD display 900 can be used to achieve higher image resolution with thesame number of active pixels in the LCD panel 950 as in a conventionalLCD display.

The above surface plasmon filters having two coupled metal-dielectricinterfaces usually produce narrow bandwidth in either reflection ortransmission with a Gaussian-like spectral profile. In certainapplications, a broad band filter is desirable.

For example, many display systems use a white light source to generatedesired primary colors by color filtering. If the bandwidth of the colorfilters is very narrow, only a small fraction of energy near the desiredprimary color wavelengths is used and the rest is rejected. Althoughthis may produce highly pure primary colors, the light utilizationefficiency is low. For certain display applications that require highdisplay brightness and low power consumption (e.g., portable computers),broad band color filters may be preferred.

A notch filter is an example of such a filter which has a relativelyflat center transmission or reflection region and sharp cut-off edges.Four, six or more metal-dielectric interfaces may be coupled to form amulti-layer surface plasmon filter to achieve a “notch” spectral profilein transmission or reflection.

FIG. 10 shows one embodiment 1000 of a multi-layer surface plasmonfilter having six metal-dielectric interfaces. Four metal films 1010 a,1010 b, 1010 c, 1010 d and three dielectric layers 1020 a, 1020 b, 1020c are alternatively stacked relative to one another to form sixsymmetric metal-dielectric interfaces. The metal films are sufficientlythin so that the evanescent electromagnetic waves can penetratetherethrough. The dielectric layers are also thin and have an opticalthickness on the order of a wavelength.

As an example, the dielectric layers may be formed of an electro-opticmaterial such as a liquid crystal. A voltage may be applied to the twoouter metal films 1010 a and 1010 d to change the optical thickness ofeach of the three dielectric layers 1020 a, 1020 b, and 1020 c. In thisconfiguration, the device effects three capacitors connected in series.The electric fields in the dielectric layers are essentially the same.The applied voltage changes the index of refraction of each dielectriclayer and thereby the optical thickness. This changes the transmissionwavelength.

FIG. 11 shows calculated reflection spectrum of the filter 1000 havingthree identical liquid crystal layers of 185 nm thick, silver films 1010a and 1010 d of 20 nm thick and silver films 1010 b and 1010 c of 40 nmthick. Reflection curve 1 represents the reflection spectrum fortransmitting in blue-green region when no voltage is applied across thesilver films 1010 a and 1010 d. When a voltage is applied to increasethe optical thickness of each dielectric layer, the transmission windowshifts toward a longer wavelength. Curves 2, 3, 4, and 5 respectivelyrepresent reflection spectra for an voltage-induced increase in theindex for 0.1, 0.2, 0.3, and 0.4.

Comparing to the transmission profiles of surface plasmon filters havingtwo coupled metal-dielectric interfaces (e.g., FIGS. 3A and 3B), thespectral profile of the multi-layer filter 1000 has a wider bandwidthand a relatively flat region in the central region of the reflection.Such profile can be achieved by properly configuring the dielectriclayers and the metal films. In general, the flatness of the “valley” inthe reflection spectrum or “peak” in the transmission spectrum can beimproved by increasing the number of layers.

FIG. 12 is a flowchart showing the design process of a multi-layersurface plasmon notch filter. If a desired reflection or transmissionprofile is not achieved by modifying the thickness values of thedielectric layers and the metal layers, the number of layers may befurther modified.

The tunable notch filter 1000 in FIG. 10 can be used to form areflective flat panel display. Consider a color filtering device havingtwo tunable notch filters F1 and F2 used in combination in reflectionmode as shown in FIG. 13A. Each notch filter is configured to transmit abandwidth that covers about two thirds of the visible spectrum. Aunpolarized white beam can be polarized by using a polarizer so that theinput beam to the first filter F1 is p-polarized. The first filter F1filters the p-polarized input beam to produce a first reflected beam ata first wavelength determined by the voltage applied thereto. The secondfilter F2, disposed relative to the first filter to receive the firstreflected beam as a p-polarized input, filters the first reflected beamto produce a second reflected beam. The reflections from these two notchfilters F1 and F2 can be combined to produce any visible color and greyscale by controlling the applied voltages to shift the reflectionspectral regions relative to each other.

Several examples for generating different colors and grey scales byusing the above color filter device are illustrated in FIG. 13A. In thefirst chart in FIG. 13A, the notch filter F1 is tuned to transmit lightfrom 400 nm to 600 nm and reflect red and other wavelengths while thenotch filter F2 is tuned to transmit the IR spectrum and to reflect allvisible light. This produces red light by reflecting a white beam offthe notch filters F1 and F2. Different red grey scales can be generatedby shifting the filters F1 and F2 to partially overlap with each otherwith different degrees in the red region. For example, shifting thefilter F2 toward the visible region while maintaining the filter F1 atthe spectral position shown would reduce the brightness of the red. Whenthe F2 is at a position to transmit green and red and reflect othercolors, the reflection becomes black (no reflected light). If thetransmission windows are completed shifted out of the visible region,the reflection is white.

The second and third charts in FIG. 13A show generation of green andblue colors, respectively, in the reflected light by using the two notchfilters F1 and F2. The fourth chart in FIG. 13A shows a dark green colorby partially overlapping the transmission windows in the green-yellowregion (approximately from 500 nm to 600 nm).

The multi-layer surface plasmon filter 1000 shown in FIG. 10 can be usedto form reflective flat panel color display. FIG. 13B depicts oneembodiment. A prism array is formed of two layers of micro prisms 1310 aand 1310 b of a high-index optical material (e.g., plastic) that stackover each other and is used to provide proper optical coupling. Theinterface sections 1320 between two layers of micro prisms are formedwith multi-layers of alternating metal films and electro-opticaldielectric layers as shown in FIG. 10. Each interface section is appliedwith a control voltage so that the reflected wavelength can beindependently controlled. A polarizing layer 1330 is formed on top ofthe prism array to select p-polarized light. The angle of the microprisms is configured in such a way that a visible light beam incident inthe normal direction to the prism array surface satisfies the surfaceplasmon resonance condition. In a preferred embodiment, the prism angleis near or at 45° as shown.

Two adjacent interface sections define one color pixel 1340. An incidentray 1350 is reflected by the two interface sections to exhibit a desiredcolor and grey scale when their transmission windows are tuned at properspectral positions.

FIG. 13C shows one example of actual color display device based on theembodiment shown in FIG. 13B. A microlens array 1360 is disposed betweenthe polarization layer 1330 and the microprism array to increase theviewing angle. Each microlens in the array 1360 is located to cover onecolor pixel. An opaque mask 1370 with an array of apertures can beplaced at or near the focal plane of the microlens array 1360. Theapertures are aligned with the pixels. In operation, light incident oneach pixel is collimated by the respective microlens and then reflectedtwice in that pixel to obtain color and grey scale, and finally isspread to a divergent beam by the same microlens to form a large viewingangle. A slant incident ray 1380, for example, is refracted by acorresponding microlens and is reflected back to another direction as aray 1382.

The reflective flat panel color displays in FIGS. 13B and 13C can useambient light for illumination. Reflected light at a wavelengthsatisfying the resonance condition of the surface plasmon filter can benearly completely reflected to achieve a high efficiency.

Such reflective color displays provide an alternative to theconventional color LCD displays widely used in notebook computers andother portable devices. A color LCD display such as the active matrixLCD in a notebook computer consumes a large portion of the power supply(e.g., as high as 80%). Use of the surface plasmon reflective displaycan significantly reduce power consumption and extend the actualoperating time of a portable device such as a notebook computer whenpowered by a battery.

Another application of the surface plasmon filters is to form a tunablecolor filtering device to produce sequential colors for projectors usingwhite light sources for illumination.

FIG. 14A shows one embodiment 1400 of such a tunable color filteringdevice. Three surface plasmon filters 1410, 1420, and 1430 are arrangedrelative to one another to sequentially reflect an incident white beam1440 from the first filter 1410 to the second filter 1420 and to thethird filter 1430. The first and third filters 1410 and 1430 are placedin a plane 1404 and are displaced from each other. The second filter1420 is placed to face the plane 1404 in a position to reflect lightfrom the first filter 1410 to the third filter 1430. A high-indexoptical material 1402 is filled between the filters 1410, 1420, and 1430to provide proper optical coupling similar to the high-index prisms usedin above surface plasmon filters. Since p-polarized light is needed toexcite surface plasmon waves in each filter, a polarizer can be placedin the input path of filter 1410 to ensure that only p-polarized lightenters the device 1400.

For a given filter, if the transmission window is set for a selectedcolor, the reflection of a white incident light is the complementarycolor of that selected color. Each filter can be configured to satisfy asurface plasmon resonance to transmit a selected primary color when novoltage is applied. In addition, a proper voltage can be applied to thefilter to destroy the resonance condition so that the filter becomescompletely reflective in the entire visible spectrum. Hence, each filtercan be configured to have two states: an “on” state to transmit aselected primary color and reflect other colors when no voltage isapplied, and an “off” state when a voltage is applied to reflect allvisible colors.

A notch surface plasmon filter as shown in FIG. 10 can be used for thispurpose. The multi-layers of alternating metal films and dielectriclayers can be configured to have a transmission window at a primarycolor (e.g., red, green, or blue) with a bandwidth of about one third ofthe visible spectrum without an external voltage. A voltage can be usedto shift the transmission window out of the visible spectrum to generatethe “off” state so the filter behaves like a mirror.

Referring to FIG. 14A, the three filters 1410, 1420, and 1430 can bedesigned to transmit red, green, and blue when no voltage is applied,respectively. Therefore, when a white light beam is sent in and novoltage is applied to any of the filter, the red portion is lost byreflection at the first filter 1410, the green portion is lost byreflection at the second filter 1420, and the blue portion is lost byreflection at the third filter 1430. Hence, no light comes out. Thisproduces a “black” color. To produce a white output, a proper voltage isapplied to each of the filters to turn “off” the transmission of eachfilter so that all filters become reflective, like mirrors. Thetransmitted colored light in each filter can be absorbed by using alight absorbing material. For example, a semiconductor substrate can beused to function as both a light absorber and a heat dissipator. Anadditional heat absorber may be used to further extract the heat fromthe light absorbing material.

An output with a primary color can be produced, therefore, by applyingvoltages only to two of the filters and applying no voltage to theremaining filter. The output color, therefore, is the primary color ofthe filter without voltage. A sequence of three primary colors can begenerated by sequentially turning on and off voltages on the filters.For example, a red output can be produced by applying voltage to thefirst filter 1410 to turn off the transmission in the visible rangewhile applying no voltage to the filters 1420 and 1430. When a voltageis applied to the second filter 1420 and no voltage is applied to thefilters 1410 and 1430, the output is green. A blue output can begenerated by applying voltage only to the third filter 1430.

This allows for a full color display. The switching rate for each filtershould be at least three times of the frame rate, e.g., 180 Hz for a60-Hz frame rate.

Such a color filtering device can be used to replace a color wheel toproduce colors with a high efficiency. Since there are no moving parts,such a filter is generally more reliable than a color wheel andsimplifies the display structure.

FIG. 14B shows the reflectivity spectra of three filters based on amulti-layer design when illuminated by a white input beam. Thecomplementary colors cyan, magenta, and yellow of the primary additivecolors red, green, and blue are respectively produced. The incidentwhite beam has a half cone angle of about 6.7° and a contrast ratio upto and greater than 200:1 can be obtained with a liquid crystal materialas the electro-optic material in the multi-layer construction. FIG. 14Cis a CIE diagram to show the color purity of the primary colors producedby the filters shown in FIG. 14B. The color triangle formed by heavylines represents colors that can be produced by the surface plasmondevice. The colors that can be produced by a 27″ CRT are indicated by acolor triangle of light lines for comparison. The natural colors arealso shown.

A spatial light modulator can be combined with the filtering device 1400in FIG. 14A to modulate the intensity of the filtered output and toproduce color images. FIG. 15 shows one embodiment 1500 of such afull-color display system. A spatial light modulator 1560 such as a LCDpanel is implemented. Since the colors are sequentially produced, thereis no need to use three adjacent pixels in the light modulator 1560 toproduce a color pixel. Each pixel forms a color pixel. This increasesthe image resolution. A white light source such as a lamp 1510 is usedfor illumination. A suitable reflector 1520 is used to direct andcollimate the light. An optical relay element 1530 (e.g., a lens)couples the light from the reflector 120 to the color filters 1410,1420, and 1430. An optical integrator 1540 is used to improve theuniformity of the intensity across the beam. A polarizer 1550 ensuresthe light incident to the filters is p-polarized. A projection lens 1570is used to project the output color images to a screen for viewing ineither front projection mode or back projection mode.

The device shown in FIG. 14A may be modified to form a special flatpanel display, a “smart card” image device having an array of activepixels each capable of producing colors and intensity modulation. FIG.16A shows an exemplary smart card image device 1600 having an inputsurface 1610 and an output surface 1612. Each pixel 1620 is formed of ahigh-index dielectric material such as a high-index glass 1630 to form alight-conducting channel to guide light from the input surface 1610 tothe output surface 1612.

The light-conducting channel has two parallel surfaces 1640 and 1650.The surface 1640 is coated with either a dielectric material with anindex less than that of the dielectric material 1630 to confine lightwithin the channel by total internal reflection or a reflective coating(e.g., metal) to reflect the light back into the channel. The surface1650 is a metallic layer to form the first metallic side of coupledmulti-layer metal-dielectric interfaces 1660 that form a surface plasmontunable filter 1670 according to the notch filter shown in FIG. 10.

Each metal layer is patterned into three separate sections along thelight conducting channel so that two adjacent metal sections in eachmetal layer are insulated from each other. This structure effects threeindependent coupled multi-layer surface plasmon filters.

A thin-film transistor layer 1670 with multiple thin-film transistors(“TFTs”) is formed on the other side of the multi-layer 1660 to providecontrol voltages to the filters. Three TFTs 1672 a, 1672 b, and 1672 care shown to respectively control voltages to the three filters formedin the multi-layer structure 1660. The metal layer 1650 is a commonelectrode for all three filters and may be set at a fixed potential orgrounded. Each control voltage supplied by a respective TFT shifts thetransmission window in a way similar to the filtering operations shownin FIG. 11. This changes the spectral composition of the beam reflectedoff that filter.

In the embodiment 1600, an incident light beam is reflected six timeswithin the light conducting channel, three times by the surface 1640 andthree times by the filters 1660, before exiting the output surface 1612.The intensity (i.e., grey scale) and color (i.e., spectral composition)of the output light beam are determined by the spectral positions of thethree transmission windows of the filters. Similar to the filteringoperations by successive reflections from two multi-layer surfaceplasmon filters shown in FIG. 13A, the relative spectral positions ofthe three transmission windows determine the color of the output beamand the amount of the overlap of the transmission windows producesdifferent grey scales. At least two filters are needed in each lightconducting channel to provide full color and intensity modulation. Ingeneral, increasing the number of filters in each conducting channel canincrease the number of grey scales and the colors.

FIG. 16B illustrates the manufacturing process of the smart card imagingdevice 1600. First, a thin plate is formed, by known techniques, toinclude the layers 1640, 1630, 1650, 1660, and 1670 shown in FIG. 16A.Then, a multiplicity of such plates are stacked and attached together byusing, e.g., a suitable adhesive. Insulating spacer layers may be usedbetween the plates. The stack is sliced at a desired angle and polishedto produce multiple smart cards.

If liquid crystal is used as the electro-optical material, the layer1660 is formed with thin chambers for filling the liquid crystal. Afterslicing, the chambers in each card are filled with liquid crystal andsealed.

The above smart card device uses the surface plasmon filters to formcolor images without color separation and color fusing. Only oneprojection is needed to image the output beams from the output surfaceto a screen. This provides an efficient display with a simple structure.

Such smart card device requires addressing a TFT array in a threedimension space since at least two TFTs are required in the directionperpendicular to the card surface. For a smart card with VGA resolution,at least 480×640×2 TFTs need to be addressed. FIG. 16C shows one designfor addressing the TFTs. The connection of the row lines are straightand can be done with known techniques. The column lines are formed bythin metal strips that run through the output surface 1612. Such thinmetal strips do not significantly affect the efficiency since thescattered light from the strips is generally in the forward direction.

One application of the such smart card device is the direct-view flatpanel display by placing a screen at the output surface. The stackstructure of the smart card device eliminate many limitations in thedirect-view LCD displays such as the glass flatness and the liquidcrystal layer uniformity. Hence, large-size direct-view flat paneldisplays exceeding 30″ can be made using such technology.

Sequential light filters, such as color wheels and the device shown inFIG. 14A, transmit one color at a time. Hence, about two thirds of theenergy of an input white beam is lost. Such loss of light energy can beavoided by combining three surface plasmon tunable filters to form acolor generating device 1700 as shown in FIG. 17A.

Three prism-coupled surface plasmon tunable filters 1710, 1720, and 1730are displaced from one another to generate three primary output colorsfrom a white input beam 1702. Two reflectors 1740 and 1750 such asprisms are respectively placed between the filters 1710 and 1720 andbetween the filters 1720 and 1730 to direct the reflected beam from thefilter 1710 to the filter 1720 and the reflected beam from the filter1720 to the reflector 1730. Preferably, each filter is a multi-layersurface plasmon filter as shown in FIG. 10 and has a tunabletransmission window with a bandwidth about one third of the visiblespectral range.

In operation, the three filters 1710, 1720, and 1730 are applied withdifferent voltages to respectively transmit at three different primarycolors. When the white input beam 1702 is p-polarized, the first primarycolor component is transmitted as a first transmitted beam 1702 a at thefirst filter 1710 and the remaining is reflected and directed to thesecond filter 1720 as a p-polarized beam 1702 b. The second filter 1720transmits the second primary component color as beam 1703 a and reflectsthe rest to the third filter 1730 as beam 1703 b. The beam 1703 b isessentially comprised of the third primary component and is transmittedthrough the third filter 1730 as beam 1704 a to produce the thirdprimary color.

Hence, at any moment, the device 1700 produces three different primarycolors. Each image frame has three fields with different colors. Toproduce color images, the voltage on each filter is scrolled tosequentially produce three different primary colors at a rate threetimes of the frame rate. Thus, in a single frame, each of the threeprimary colors is scrolled through all three filters once. FIGS. 17A,17B, and 17C show one cycle of such color scrolling in a frame.

Similar to the color display system shown in FIG. 15, a spatial lightmodulator such as a LCD panel only needs one pixel to produce all threecolors. This eliminates the need for expensive miniaturized colorfilters and the requirement of using three pixels to form one colorpixel as in many conventional LCD displays. Hence, using the device inFIG. 17 can improve image resolution, simplify device structure, andreduce manufacturing cost. Since all colors are used, the efficiency ofsuch device is increased by a factor of 3 over many conventional LCDcolor displays.

All visible light in the p-polarized input is utilized in the device1700. However, if the input white beam 1702 is unpolarized like in manylamp sources, only the p-polarized portion in the visible range is usedfor image display and the other half of s-polarized light is rejected asan output beam 1704 b. FIG. 18 illustrates a surface plasmon device 1800capable of using all visible energy of a unpolarized white input beam.

The device 1800 combines two devices 1810 and 1820 as shown in FIG. 17Ato achieve the above purpose. The device 1820 is rotated with respect tothe device 1810 in such a way that the rejected s-polarized light by thedevice 1810 enters the device 1820 as p-polarized light. The device 1810is used to produce one half (e.g., 1830 a) of a display 1830 and thedevice 1820 is used to produce the other half (e.g., 1830 b).Alternatively, the output beams of the two devices 1810 and 1820 may beoverlapped to produce a display.

In the embodiment shown in FIG. 18, the device 1820 is formed by adevice identical to the device 1810 but is rotated 90° so that as-polarized light with respect to the device 1810 becomes p-polarized indevice 1820. In addition, three reflectors (e.g., prisms) 1822, 1824,and 1826 are added to respectively direct the transmitted light in eachfilter in the device 1820 in a direction perpendicular to the respectiveincidence plane so that the output beams 1820 a, 1820 b, 1820 c from thedevice 1820 are parallel to the output beams 1810 a, 1810 b, 1810 c fromthe device 1810. Since not only all three colors are used but also bothpolarizations are used, the device 1800 can achieve an efficiency by afactor of 6 over many conventional LCD color displays.

Although the present invention has been described in detail withreference to the preferred embodiments, various modifications andenhancements may be made. For example, the coupling prisms, e.g., prisms230 a and 230 b, may be made with angles other than an apex angle of 90°as illustrated; a polarizing element may be disposed in the path of theincident beam to change the input polarization to the p-polarization;the surface plasmon filter 930 in the LCD display may be grating coupledor have a micro-prism configuration; the LCD panel 950 may a digitalmirror array. In addition, the air gaps may be a gap with a gas otherthan the air.

For another example, although all the embodiments disclosed changeeither the thickness or the index of refraction of the center dielectriclayer to tune the transmission wavelength, the incident angle of theinput light may also be adjusted to change the surface plasmon resonancecondition, thereby changing the transmission wavelength. The incidentangle may be changed with an angle adjustment device by either adjustingthe direction of the input beam or adjusting the orientation of thesurface plasmon filter relative to the input beam. In implementation ofsuch angular tuning scheme, the index of refraction and the spacingbetween the two metal-dielectric interfaces may be fixed so that theincident angle is the only tuning parameter. Alternatively, the incidentangle and either or both of the index of refraction and the spacingbetween the two metal-dielectric interfaces may be adjusted to tune thetransmission wavelength.

In various color filtering and display applications, coupled multilayersurface plasmon filters such as the one in FIG. 10 are used to take theadvantages of the square-like notch transmission window and theconfigurable transmission bandwidth obtained from proper configurationsof the multiple metal-dielectric layers. However, the non-coupledabsorbing surface plasmon filters having a single metal-dielectric layeras disclosed in the incorporated U.S. Pat. Nos. 5,570,139 and 5,451,980can also be used. Although configuring the metal layer by combiningdifferent metal films can only offer limited flexibility in changing theabsorption spectral profile, the simplicity in the structure of suchfilters provides easy construction of many of the above devices.

For example, if the surface plasmon filters in the devices of FIG. 13are implemented by non-coupled surface plasmon filters, there is no needto eliminate the unwanted transmitted light since. In forming the device1600, each pixel 1620 can be significantly simplified by using suchnon-coupled surface plasmon filters. FIG. 19A shows such an alternativeconstruction of the pixel 1620. Only three layers are needed to formeach filter: a metal layer 1650, a liquid crystal layer 1690 and the TFTlayer 1670. This simplifies the manufacturing of each plate for formingthe stack. FIG. 19B shows that each plate can be a simple high-indexglass plate with one side formed with TFTs and the other side coatedwith a metal layer. Such plates are spaced by spacers when forming thestack so that the liquid crystal can be filled in the space provided bythe spacers after slicing.

These and other variations and modifications are intended to beencompassed by the appended claims.

What is claimed is:
 1. An optical device, comprising: a first array ofprisms formed of a transparent material, each prism having at least afirst prism facet and a second prism facet intersecting said first prismfacet at a prism apex; at least one metal layer and an electro-opticaldielectric layer contacting each other to form a metal-dielectricinterface on each prism facet, wherein said dielectric layer has arefractive index less than a refractive index of said first and secondarrays of prisms, and wherein said metal-dielectric interface generatesa surface plasmon wave in response to a p-polarized input light beam totransmit light at a selected wavelength within a bandwidth according toa control voltage from said metal layer to said dielectric layer andreflects light of other wavelengths; and a second array of prismssubstantially identical to said first array, disposed to engage to saidfirst array in such a way that each prism facet in said second arrayconformably contacts said metal-dielectric interface formed on arespective prism facet of said first array, wherein said second arraycouples an input light beam to said metal-dielectric interface and thesaid first and second arrays are configured so that said input lightbeam is respectively reflected once by metal-dielectric interfaces ontwo adjacent prism facets of said first array to produce a desired colorand a desired grey scale.
 2. The device as in claim 1, furthercomprising a polarization layer disposed relative to said second array.3. The device as in claim 2, further comprising a lens array disposedbetween said polarization layer and said second array, wherein each lensin said lens array is positioned over two adjacent prism facets of twodifferent prisms in said first array.
 4. A device, comprising, a firsttunable optical filter disposed to receive an input light beam toproduce a first reflected beam and configured to include at least onemetal layer and one electro-optical dielectric layer which form ametal-dielectric interface to generate a surface plasmon wave inresponse to a p-polarized input beam to transmit light at a firstselected wavelength within a first bandwidth according to a firstcontrol voltage and to reflect light at other wavelengths; a secondtunable optical filter disposed relative to said first tunable opticalfilter to receive said first reflected light beam, said second tunableoptical filter having at least one metal layer and one electro-opticaldielectric layer to form a metal-dielectric interface that generates asurface plasmon wave in response to a p-polarized input beam to transmitlight at a second selected wavelength within a second bandwidthaccording to a second control voltage and to reflect light at otherwavelengths, wherein said second optical filter reflects said firstreflected light beam to produce a second reflected light beam with aselected color different than a color of said input light beam, andwherein each of said first and second tunable optical filters comprises:a first coupling element having a first coupling surface and operable tocouple a p-polarized input electromagnetic wave to have a propagationcomponent parallel to said first coupling surface; a first metallic filmformed on said first coupling surface and operable to produce freeelectron oscillations in response to excitation of said p-polarizedinput electromagnetic wave, wherein said first metallic film isconfigured to allow penetration of evanescent fields; a first dielectriclayer having a first surface in direct contact with said first metallicfilm to form a first metal-dielectric interface and a second surfaceopposing said first surface; a second metallic film substantiallyidentical to said first metallic film and formed on said second surfaceof said first dielectric layer to form a second metal-dielectricinterface that is substantially symmetric to said first metal-dielectricinterface with respect to said first dielectric layer; a seconddielectric layer having a first surface in direct contact with saidsecond metallic film to form a third metal-dielectric interface and asecond surface opposing said first surface; a third metallic filmsubstantially identical to said second and first metallic films andformed on said second surface of said second dielectric layer to form afourth metal-dielectric interface that is substantially symmetric tosaid third metal-dielectric interface with respect to said seconddielectric layer; and a second light coupling element configured to havea second coupling surface which is in direct contact with said thirdmetallic film and operable to couple an electromagnetic wave propagatingin said third metallic film along said second coupling surface into anoutput electromagnetic wave, wherein said first, second, third, andfourth metal-dielectric interfaces are operable to support surfaceplasmon waves and to transfer energy of a selected spectral component insaid p-polarized input electromagnetic wave from said first couplingsurface to said second coupling surface by coupling a selected surfaceplasmon mode on said first metal-dielectric interface to said fourthmetal-dielectric interface.
 5. The device as in claim 4, wherein saidfirst and second dielectric layers are formed of an electro-opticmaterial and said first and third metallic films are applied withdifferent voltages to change an optical thickness of said dielectriclayers to tune a center wavelength of said selected spectral component.6. The device as in claim 4, wherein said first and second dielectriclayers have a thickness on the order of about one wavelength of saidinput electromagnetic wave.
 7. The device as in claim 4, wherein saidfirst and second coupling elements comprise prisms which are made of adielectric material having a dielectric constant higher than adielectric constant of said first and second dielectric layers.
 8. Thedevice as in claim 4, further comprising a spatial light modulatorpositioned to receive said second reflected beam from said secondoptical filter, said spatial light modulator having an array oflight-modulating pixels to modulate said second reflected beam toproduce an image.
 9. The device as in claim 8, wherein said spatiallight modulator includes a liquid crystal panel.
 10. The device as inclaim 4, further comprising an optical integrator positioned in anoptical path of said input light beam to increase uniformity of aspatial intensity distribution of said input light beam.